1. Field of the Invention
This invention relates to a method and apparatus for providing efficient handoffs between cells and/or sectors within a cellular mobile data communication network.
2. Description of Related Art
It has long been a goal of communications engineers to establish a mobile communication network that would allow an individual to maintain wireless communications with others. That goal is being realized today by a mobile cellular communication system, commonly referred to as Advanced Mobile Phone Service (AMPS), in which an area is geographically divided into cells. A typical cell in a cellular system may be sectored or omni-directional. In a sectored cell, the coverage may be divided among several antennae that serve different regions of the cell. Typically, three to six antennae are used within a cell. A Base Station associated with each cell sector controls airlink access to mobile units (which are typically mobile/cellular phones).
FIG. 1 illustrates a number of sectored cells 102 arranged to cover a relatively large geographic area. FIG. 2 illustrates a single sectored cell 102. Each cell 102 typically has three sectors 103, each of which is serviced by a corresponding one base station 101a, 101b, 101c having an independent antenna. Each sector 103 has a "footprint" 105 (i.e., an area that is within the range of the base station 101 for both transmit and receive signals), which may differ in size and shape from sector to sector. As depicted in FIG. 2, equal received power may be observed at a mobile end system 109 (such as a mobile telephone or mobile end-system as defined by the specification for CDPD Release 1.0, Jul. 19, 1993) at locations all along the broken line 105, which represents the footprint boundary. Typically, a number of cells 102 are arranged in proximity to one another, such that the sectors 103 of adjacent cells 102 overlap. Such overlapping of sectors 103 of adjacent cells 102 generally ensures that a mobile end system 109 may maintain contact with at least one base station 101 from any location.
Although overlapping of sectors 103 is desirable, it is not desirable for a mobile end system 109 physically within a first sector 103 to establish a link with the base station 101 of another sector or cell. AMPS prevents this as follows. Each sector 103 of a cell 102 has a scanning (or location) receiver that can monitor the quality of signals from a mobile end system 109 being served by other sectors or cells. One measure of signal quality is the received signal level, typically indicated by a parameter called Received Signal Strength Indication (RSSI). "Handoffs" (i.e., transfer of control of a call from one base station to another with little or no disturbance to the telephone user 109) in AMPS are controlled by the Mobile Telephone Switching Office (MTSO) 111. The term "controlling base station" will be used to refer to the base station that is currently serving the call. The MTSO 111 compares differences in signal quality as measured at various scanning receivers (located at various base stations under control of the MTSO 111) and makes a decision to handoff when the controlling base station has a significantly lower measure of mobile phone signal quality than another base station. The call is then transferred to the control of the base station best suited to supporting that call. In a typical AMPS system, two signal level thresholds are assigned: a primary and a secondary (emergency) handoff threshold. When the received signal level of an AMPS mobile end system 109, such as a cellular mobile phone, is below the primary threshold, a Mobile Telephone Switching Office (MTSO) 111 in communication with each of the base stations 101 within a particular local area signals scanning receivers at other cells 102 to monitor the received signal of that mobile end system 109. When the scanning receiver at another sector/cell receives the mobile end system 109 in question at a higher power than the controlling sector 103, that sector or cell sends an indication to the MTSO 111, which can, via the base station 101, command the mobile end system 109 to change channels to a channel assigned to the sector/cell having the higher received signal power. This results in more stringently defined cell boundaries than can be obtained if cell transfers are based only on the strength of the signal received by the base station 101 through which a cellular phone is connected. The secondary threshold is typically used to determine the level below which a call will not be supported by the base station 101 through which the cellular phone is connected. Usually, violation of the secondary threshold will lead to the call being dropped.
In addition to the AMPS system, which allows wireless voice communication using a mobile cellular phone, a system commonly referred to as Cellular Digital Packet Data (CDPD) allows wireless digital packet data to be communicated using a mobile end system coupled to a computer. In accordance with the specification for CDPD (CDPD Release 1.0, Jul. 19, 1993), data is packetized and transmitted on AMPS channels that are not being used by AMPS (i.e., no AMPS base station is transmitting on the channel). This may be done by dedicating channels for use by CDPD or by making CDPD channels share AMPS allocations. FIG. 3 is a simplified block diagram which illustrates the relationship of the AMPS system to the CDPD system within a base station 101. Typically, an AMPS transmitter 305 is coupled to a from end gain amplifier 303 which amplifies the outgoing RF signal before the transmitting the RF signal through the antenna 301. As the signal is output by the AMPS transmitter 305, the RF signal is coupled to a coupling pad 307 which provides a portion of the RF signal to a CDPD "sniffer" circuit 309. The CDPD sniffer circuit 309 detects the RF signal and causes a CDPD receiver/transmitter 310 to cease transmitting CDPD signals. In a shared allocation scheme, when an AMPS signal is detected by a sniffer 309, the CDPD signal "hops" to another unused channel. Thus, the CDPD data communication network is overlaid on AMPS. That is, CDPD base stations 101 that are co-situated with AMPS base stations 101 use at least some of the same frequencies in each cell or sector, and the cells 102 for the two systems preferably have the same geographical footprints.
Thus, in CDPD, various subscribers (mobile users who transmit through a CDPD base station) can gain access to services through mobile end-systems 109. Each base station 101 acts as a relay and connects subscribers to a Mobile Data Intermediate System (MDIS) 112. The MDIS 112, in turn, relays and connects subscribers to other networks. In a packet based communication network, such as CDPD, a typical design may require subscribers to contend for access to an airlink. In such a system, the forward link (from base station 101 to mobile end system 109) may be continuously active, while the reverse link (from mobile end system 109 to base station 101) is intermittent. Subscribers in such a system may be required to synchronize access to the reverse channel with a synchronization word or a clock transmitted periodically on the forward channel. One multiple access scheme is commonly referred to as Digital Sense Multiple Access (DSMA), the basic principle of which is similar to Carrier Sense Multiple Access with Collision Detection (CSMA/CD). In such systems, it is very convenient to assign the responsibility for determining when to handoff a call to the mobile end system 109. Due to the fact that the forward channel can be continually monitored by each subscriber, it helps to have a radio resource management entity (RRME) (which may be a series of functions that do not have a physical form) in the mobile end system that is charged with, among other tasks, executing a handoff and determining whether and when to do so.
Even in systems that have packet based forward channels, as well as reverse channels, it greatly helps to assign responsibility for cell transfers to the mobile end system. This is because in data communication systems, management of the radio interface may be a relatively small overhead for the mobile end system, whereas assignment of that responsibility to the base station 101 may prove especially burdensome.
Various conditions are typically defined under which a handoff or a cell transfer is supposed to occur. In accordance with the CDPD specification, cell transfers are initiated by the mobile end system and may occur under some or all of the following conditions:
(1) the received signal strength indication (RSSI) averaged over time (RSSI.sub.-- TIME) at the mobile end system drops below a threshold called RSSI.sub.-- THRESHOLD. PA1 (2) the block error rate (BLER) averaged over time (BLER.sub.-- TIME) at the end system exceeds a threshold called BLER.sub.-- THRESHOLD. PA1 (3) the symbol error rate (SER) averaged over time (SER.sub.-- TIME) at the end system exceeds a threshold called SER.sub.-- THRESHOLD. In general, the term "symbol" refers to one or more related bits. Thus, the definition of a symbol for any particular system may be unique. For example, the symbol in one system may be defined as an 8-bit word, whereas in another system, a symbol may be defined as a 32-bit character.
The above threshold list is typically unique for any given cell or sector. All, or part, of the above list may be transmitted as a cell configuration message from the controlling base station 101 or an MDIS 112, as defined in the CDPD specification, for example. The last two conditions serve to ensure a good communication link between the base station 101 and the mobile end system, whereas the first condition is more significant in relation to causing a cell transfer based on the distance from the serving base station 101.
Carriers deploying CDPD have expressed a desire to have CDPD's coverage emulate AMPS very closely. One of the requirements that this poses on the system is that cell footprints or geometry be the same in the two systems. However, strict adherence to the CDPD specification does not allow this, since AMPS requires that handoffs be initiated by the base station 101 and the CDPD specification requires that handoffs be initiated by the mobile end system 109. Furthermore, a failing in the use of scanning receivers as the sole means for determining when to change channels is that in systems with an intermittent or bursty reverse channel, a mobile end system may not transmit very often, and therefore, cannot be monitored very efficiently at the base station 101. Hence, a scanning receiver may not correctly identify a mobile end system 109 as a handoff candidate in a reasonable period of time.
In one system (described in a patent issued to Scotton, et al., U.S. Pat. No. 4,829,519 issued on May 9, 1989) bit or symbol error rates of messages received by the mobile end system 109 from the base station 101 are assessed to determine when to cause the mobile end system to change channels to a channel assigned to another cell (execute a handoff). However, in accordance with Scotton, the point at which a mobile end system determines that a handoff is to occur is not dependent upon the distance of the mobile end system from a base station. That is, only after the signal strength falls below a particular value will the error rate be effected. By that time, the mobile end system may be well outside the desired cell (the cell as defined for the associated AMPS) or alternatively, the mobile end system may be in an area of poor signal coverage well within the desired cell, such as indoors, in an obstructed location, or at a null created by multipathing which causes degenerative interference. Therefore, a cell defined for a digital packet data system (such as CDPD) may have a substantially different size from the desired cell (e.g., a cell defined for a host AMPS system on which the CDPD system is overlaid). Further, Scotton is provided in the context of an AMPS system generally. Therefore, Scotton assumes that error rates worse than an exit threshold indicate that a handoff should occur, neglecting the possibility that a hop has occurred to a channel within the same cell. Still further, Scotton attempts to create a link to the first channel which is above an entry threshold without further investigation to determine whether other channels may have superior performance. Furthermore, while Scotton discloses an entry threshold and an exit threshold, these thresholds are used with respect to different signals. That is, the exit threshold is used in association with the signal that is being received to determine when the mobile end system has exited a cell, whereas the entry threshold is used in association with the signal to which the system will switch to determine whether the mobile end system has entered another cell. It can be seen from Scotten that using one threshold to determine when to cause a handoff to occur can result in a cell that covers either more or less area than desired.
Accordingly, it would be desirable to provide a system that may be used in a communication network, such as CDPD, to ensure that cell footprints may be well defined.